Updated: March 2011
Provides a means for accessing a managed object from unmanaged memory.
Assembly: mscorlib (in mscorlib.dll)
The structure is used in conjunction with the GCHandleType enumeration to create a handle corresponding to any managed object. This handle can be one of four types: Weak, WeakTrackResurrection, Normal, or Pinned. Once allocated, you can use a to prevent the managed object from being collected by the garbage collector when an unmanaged client holds the only reference. Without such a handle, the object can be collected by the garbage collector before completing its work on behalf of the unmanaged client.
You can also use the to create a pinned object that returns a memory address and prevents the garbage collector from moving the object in memory.
When the handle goes out of scope you must explicitly release it by calling the Free method; otherwise, memory leaks may occur. When you free a pinned handle, the associated object will be unpinned and will become eligible for garbage collection, if there are no other references to it.
The following example shows an App class that creates a handle to a managed object using the GCHandle.Alloc method, which prevents the managed object from being collected. A call to the EnumWindows method passes a delegate and a managed object (both declared as managed types, but not shown), and casts the handle to an IntPtr. The unmanaged function passes the type back to the caller as a parameter of the callback function.
Imports System Imports System.IO Imports System.Threading Imports System.Windows.Forms Imports System.Runtime.InteropServices Imports System.Security.Permissions Public Delegate Function CallBack(ByVal handle As Integer, ByVal param As IntPtr) As Boolean Module LibWrap ' passing managed object as LPARAM ' BOOL EnumWindows(WNDENUMPROC lpEnumFunc, LPARAM lParam); <DllImport("user32.dll")> _ Function EnumWindows(ByVal cb As CallBack, ByVal param As IntPtr) As Boolean End Function End Module 'LibWrap Module App Sub Main() Run() End Sub <SecurityPermission(SecurityAction.Demand, UnmanagedCode:=true)> _ Sub Run() Dim tw As TextWriter = System.Console.Out Dim gch As GCHandle = GCHandle.Alloc(tw) Dim cewp As CallBack cewp = AddressOf CaptureEnumWindowsProc ' platform invoke will prevent delegate to be garbage collected ' before call ends LibWrap.EnumWindows(cewp, GCHandle.ToIntPtr(gch)) gch.Free() End Sub Function CaptureEnumWindowsProc(ByVal handle As Integer, ByVal param As IntPtr) As Boolean Dim gch As GCHandle = GCHandle.FromIntPtr(param) Dim tw As TextWriter = CType(gch.Target, TextWriter) tw.WriteLine(handle) Return True End Function End Module
Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune
The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.